The current trend in miniaturizing and reducing the manufacturing costs of personal portable communication equipment, such as cellular telephones, has prompted engineers to study the design of the antennas within the portable communications equipment. See J. Rasinger, et al., A New Enhanced-Bandwidth Internal Antenna for Portable Communications, 40.sup.TH IEEE VEHICULAR TECH. CONF., 1990; I. G. Choi, et al., UHF Tapered Bent-Slot Antenna for Small Sized Portable Phones, 42.sup.ND IEEE VEHICULAR TECH. CONF., P. 9-12, (1992); X. Z. Li, et al., Research Report on 1.8 GHz Foldable Hand-Machine Antenna; E. Onegreau, EEsof User's Group Meeting, SONNET EM USER'S MANUAL, ver. 2.4, p. 85, 1993; U.S. Pat. No. 4,401,988; U.S. Pat. No. 4,965,605. Some conventional antenna designs which have been commercialized include short wire antennas, small loop antennas and normal mode helical antennas.
Perhaps the greatest challenge to miniaturizing the antennas is maintenance of the frequency bandwidth of the antenna. Generally speaking, bandwidth narrowing renders the communication more susceptible to degradation as a result of changes in the environment. Aside from performance issues, it is also desirable to reduce the cost of manufacturing the antenna, and to reduce the complexity of antenna manufacture.
FIG. 1 shows a first conventional antenna 10 referred to as a "plane" dual-L antenna taught by X. Z. Li, et al., Research Report on 1.8 GHz Foldable Hand-Machine Antenna. As shown, the antenna includes a ground plane 12, and two L-cross sectioned resonant units 14 and 16 connected to the ground plane 12. The bandwidth is adjusted by the coupling across the opening between the two resonant units 14 and 16. The field patterns for the antenna 10 are illustrated in FIGS. 2, 3 and 4. FIG. 5 shows the variation of the reflection coefficient s.sub.11 of the antenna 10 in relation to frequency. As shown, the antenna 10 has a large bandwidth.
The antenna 10 is referred to as a "plane" antenna because the conductors of the resonant units 14,16 are in the same planes; the portions 14a and 16a are in the same plane and the portions 14b and 16b are in the same plane. The dimensions of the antenna 10 are as follows: L1=2.8 cm, w1=0.45 cm, L2=5.27 cm, w2=0.45 cm, h34=0.5 cm, w34=0.45 cm, h5=0.5, L6=4.0 cm, w6=1.0 cm, s1=s2=0.1 cm. A problem with the antenna 10 is that it takes up a large amount of volume (i.e., 5.27 cm.sup.3) and a large area (i.e., 2.8 cm.sup.2). In addition, the antenna 10 must be constructed using a special metal work processing that cannot be done automatically, i.e., must be done manually. Furthermore, the antenna 10 requires a special copper on aluminum alloy coating to render the antenna vibration proof.
FIG. 6 illustrates a second antenna 20 referred to as a "coupled microstrip patch antenna." The coupled microstrip patch antenna 20 includes plural, e.g., three, resonator patches 22, 24 and 26 which are all located in the same plane. Illustratively, the antenna shown in FIG. 6 is designed for 2.4 GHz. FIG. 7 illustrates the variation of the reflection coefficient in relation to frequency. As shown, the bandwidth of the antenna 20 is limited to about 1%. Nevertheless, such a narrow bandwidth is useful for beam antennas, e.g., in radar arrays.
FIG. 8 illustrates a multi-layered microstrip patch antenna 30 disclosed in U.S. Pat. No. 4,401,988. A feed pin 31, of a coaxial cable 32 is connected to a radiating element patch 33. The radiating element patch 33 is affixed to a dielectric substrate 34 which separates the radiating element patch 33 from a parasitic element 35. The parasitic element 35 is affixed to another dielectric 36 which separates the parasitic element 35 from a ground plane layer 37. The coupling effect between the radiating element patch 33 and the parasitic element 35 enhances the radiation at angles closer to the ground plane. Compare FIG. 10, which shows a field pattern for the single layer microstrip patch antenna 20 of FIG. 6, to FIG. 9, which shows a field pattern for the multi-layered microstrip patch antenna 30 of FIG. 8. Note the field pattern as the elevation increases from ground level beyond 45.degree.. The maximum field value occurs at 90.degree. from ground level, i.e., at right angles to the patches. When the coupled microstrip patch antenna 20 is arrayed, the beam is typically even narrower.
The problem with the coupled microstrip patch antenna is the extremely large area which it occupies, i.e., on the order of 30 cm.sup.2. In addition, the coupled microstrip patch antenna produces a highly directional beam. In small portable communications devices, it is desirable for an antenna to achieve the contrary effect--to produce an omni-directional field pattern. This ensures good reception regardless of how the antenna is oriented in regard to the other transceiver. Furthermore, the coupled microstrip patch antenna must be assembled manually.
It is an object of the present invention to overcome the disadvantages of the prior art.